An organic electroluminescent element (hereinafter referred to as an “organic EL element”) is a self-light-emitting element including a light emitting layer made of an organic compound between a cathode and an anode which are opposite to each other. When a voltage is applied between the cathode and the anode, light is emitted due to excitons that are generated when electrons injected into the light emitting layer from the side of the cathode and holes injected into the light emitting layer from the side of the anode recombine in the light emitting layer.
It is known that, in order to increase efficiency of an organic EL element, when a structure in which organic compounds with different carrier transporting properties are laminated, electrons and holes are injected into a cathode and an anode, respectively, in a balanced manner is provided, and a layer thickness of an organic layer interposed between the cathode and the anode is set to 0.2 μm or less, sufficiently practical high luminance and high efficiency are obtained at an applied voltage of 10 V or less, 1000 cd/m2, and an external quantum efficiency of 1% (for example, refer to Non Patent Literature 1).
In addition, it is described in Non Patent Literature 1 that, when the layer thickness of all organic layers interposed between the cathode and the anode is set to 1 μm or less, it is possible to provide a device that can emit light at a lower applied voltage. It is described that, when the layer thickness of the organic layer is in a range of 0.1 μm to 0.5 μm, an electric field (V/cm) useful in emitting light at an applied voltage of 25 V or less is preferably obtained (for example, refer to Patent Literatures 1 to 6).
The organic EL element has been developed based on the element structure described in Non Patent Literature 1.
In addition, as the element structure of the organic EL element, an organic EL element having a tandem structure in which a light emitting unit including at least one light emitting layer is set as one unit and a plurality of light emitting units are laminated to be connected in series between a cathode and an anode is developed (for example, refer to Patent Literatures 7 and 8).
The organic EL element having the tandem structure has been focused on as a technology through which it is possible to obtain a long lifespan, high luminance, and uniform light emission in a large area which are not possible to obtain in the structure of the organic EL element described in Non Patent Literature 1 described above in which a high current is necessary although the voltage is low.
Further, an organic EL element having a multi-photon emission (MPE) structure in which an electrically insulating charge generating layer (CGL) is arranged between a plurality of light emitting units is developed (for example, refer to Patent Literatures 9 and 10). In the organic EL element having the MPE structure, when a voltage is applied between a cathode and an anode, charges in a charge transfer complex move toward the cathode and the anode, and thus holes are injected into one light emitting unit positioned on the side of the cathode with sandwiching the charge generating layer, and electrons are injected into the other light emitting unit positioned on the side of the anode with sandwiching the charge generating layer. Accordingly, since light beams are emitted from the plurality of light emitting units at the same time at the same amperage, it is possible to obtain current efficiency and external quantum efficiency corresponding to the number of light emitting units.
The above-described organic EL element has characteristics by which it is possible to switch light emission at a high speed and it is possible to obtain a simple and thin element structure. The organic EL element is applied to a display device of, for example, a mobile phone and an in-vehicle device, utilizing such excellent characteristics. Further, in recent years, for example, a backlight of a liquid crystal display and a lighting device such as a general lighting device utilizing a thin surface light emission characteristic have been focused on.